The present invention relates to a vacuum switch.
22/33 kV and 66/77 kV grade special high-voltage transformers of the prior art have involved the problems of construction costs caused by the high cost of the necessary land, of insulation and safety problems when the charging parts become soiled and with noise when the switch is operated. Accordingly there has been progress with the size reduction and sealing of the switches and air-insulated switches have been replaced by gas-insulation-type gas-insulated switchgeares and cubicle-type gas-insulated switchgeares.
In the former type of gas-insulated switchgear. the interrupters, disconnecting switches and conductors connected to these are contained in a sealed metal tubular container and this tubular container is filled at high pressure with sulfur hexafluoride (SF6) gas as an insulating gas to achieve size reduction and sealing.
By contrast, the latter type, the cubicle-type gas-insulated switchgear, has been developed to have higher reliability, safety and ease of maintenance and inspection than the former gas-insulated switchgear, together with reducing the space required for installation, shortening construction time and meeting the demand for harmony with the environment surrounding the site of installation.
Thus, in the cubicle-type gas-insulated switchgear, all the electrical devices described above and connections and conductors are contained within a case that is filled with insulating gas at slightly greater than atmospheric pressure, and the interior is divided into gas zones for each circuit, which facilitates maintenance after installation.
Cubicle-type gas-insulated switchgears, which in their external appearance are very similar to the atmosphere-insulated metal enclosure switchgear formerly installed, have come into use to meet contemporary requirements, as described above.
FIG. 1 shows a right-side view (without the right-side plate) of one example of a cubicle-type gas-insulated switchgear, in this case a load-receiving board.
In FIG. 1, front door 14A is attached to the front surface of case 13, the external periphery of which is encased so as to be air-tight by mild steel sheet.
Within case 13, U-shaped partition 15A is welded at its top edges to the ceiling of the case, vertical partition 15B is attached in an air-tight manner somewhat forward of the center of partition 15A and bus-bar partition 15C, which is formed in an L shape, is attached in an air-tight manner to the back of vertical partition 15B.
As a result of this, a U-shaped air insulation cubicle 13a is formed in front of, behind and below partition 15A, interrupter cubicle 13b is formed in front of partition 15B, an L-shaped load-receiver cubicle 13c is formed between the bottom of partition 15B and partition 15A and a small busbar cubicle 13d is formed above load-receiver cubicle 13c. 
Interrupter cubicle 13b, load-receiver cubicle 13c and bus-bar cubicle 13d are filled with SF6 gas 25, as described above.
A vacuum interrupter 16, with a vacuum valve as the switch part, is housed in interrupter cubicle 13b and operating mechanism part 17, which is fitted with a wheel and which operates the switch part of vacuum interrupter 16, is housed at the bottom of this vacuum interrupter 16 so as to be capable of being extracted into air-insulated cubicle 13a. 
Disconnecting switch operating mechanism 22 is attached to the front surface of partition 15A at the front of vacuum interrupter 16 and an operating rod (not shown), which projects to the rear from this disconnecting switch operating mechanism 22, passes through partition 15B, at the rear of interrupter cubicle 13b, so as to be air-tight.
Insulating spacers 19A (upper) and 19B (lower) are fitted through partition 15B. The front end of the upper insulating spacer 19A is connected by connecting conductor 18A, covered by a shield tube, to the upper terminal of vacuum interrupter 16 and the front end of lower insulating spacer 19B is connected by a short connecting conductor to the bottom terminal of vacuum interrupter 16.
In bus-bar cubicle 13d, disconnecting switch 20 is fixed to the base board, the terminal of the fixed side of disconnecting switch 20 is connected to the back of the insulating spacer 19A, in front of it, and the rear end of an operating rod (not shown), which projects rearward from disconnecting switch operating mechanism 22, as described above, is linked to the top end of lever 20a which stands at the rear of the disconnecting switch 20.
The movable side terminal shown at the front of the top end of lever 20a of disconnecting switch 20 is connected by a connecting conductor to the bottom end of insulating bushing 21 which passes vertically through a thick attachment sheet welded to the ceiling sheet of bus-bar cubicle 13d. 
The top end of voltage-detecting insulator 23 is fixed to the rear lower surface of the base sheet of bus-bar cubicle 13d and the rear end of connecting conductor 18B, covered by a shield tube, is connected to the terminal at the lower end of voltage-detecting insulator 23 and the front end of connecting conductor 18B is connected to the rear part terminal of lower insulating spacer 19B, in front of it.
The front end of the short connecting conductor 18C, covered with a shield tube, is connected to the rear of the terminal at the bottom end of the voltage-detecting insulator 23 and the rear end of connecting conductor 18C is connected to the front end of cable head 26 which is fitted so as to pass through from the rear of the rear end of partition 15A.
The front end of cable head 26 is again connected to the bottom terminal of arrester 24 which is fitted so as to pass through the ceiling sheet of load-receiving cubicle 13c from above.
The top end of high-pressure cross-linked polyethylene cable 27 brought up from a pit formed, as indicated by the broken line, in the floor fitted to the case 13, is connected to the underneath of the cable head 26 and the high-pressure cross-linked polyethylene cable 27 passes through a current transformer 28 fixed to the case.
The connection part of the top of the insulation bushing 21 that is fitted through the ceiling plate of case 13, is connected, via a high-pressure cross-linked polyethylene cable (not shown) to the connection part of the top of an insulation bushing fitted through the ceiling plate of the load-receiving board (not shown) of another system fitted next to box 13.
In a load-receiving board configured in this way as a cubicle-type gas-insulated switchgear, the power fed to the load-receiving board from the exterior disconnecting switch (not shown) fitted in this transformer, via the high-pressure cross-linked polyethylene cable 27 in the pit, passes though vacuum interrupter 16 and disconnecting switch 20 and is fed from a high-pressure cross-linked polyethylene cable fitted at the top of the ceiling surface of the case 13 of the load-receiving board to the load side, via a load-dispatching board (not shown).
The SF6 which fills interrupter cubicle 13b, load-receiving cubicle 13c and bus bar cubicle 13d has arc-extinguishing capability 100-fold and a insulating capability 3-fold that of air and it is this SF6 gas that allows the case to be reduced in size.
Moreover, this is a colourless, odorless, tasteless gas with stable uninflammabity and it is also non-toxic. If, however, it is brought into contact with an arc, highly toxic degradation gases and degradation products such as SOF2, SO2, SO2F2, SOF4, HF and SiF4 are generated and special treatment and management are required to recover these degradation products and degradation gases from SF6 gas.
In FIG. 1, of the switches incorporated into the load-receiving board, the vacuum interrupter 16 extinguishes the arc inside the vacuum valve, and Bo no degradation gases of SF6 gas are generated but when in disconnecting switch 20, the loop current is interrupted when the circuit is switching or the bus-bar is switched within the insulating gas in the substation (transformer station), and arcing occurs, albeit less than with an accidental current.
Furthermore, SF4 gas is a greenhouse effect gas and thus one of the causes of global warming, with a global warming index 24,000-fold that of carbon dioxide.
Because of this, at the Third Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change (COP3), held in Kyoto, Japan in December 1997, SF6 gas was added to the gases to be reduced and there was demand for emissions to be limited.
Vacuum has been therefore considered as an insulating medium for interrupters but there are large variations in the insulating capability of vacuum.
If these variations are expressed as standard deviations, that of SF6 gas is 6-7%, whereas that of vacuum is 10-13% and this may be increased in switch conditions to reach 18%.
Moreover, since, in terms of the safety of power circuits, there is a strong requirement for the insulation of interrupters to be reliable and thus a requirement for the development of an interrupter in which the insulation reliability of the vacuum atmosphere of the prior art is further improved.
Accordingly, one object of the present invention is to provide a vacuum switch capable of meeting the demands for improvement in environmental protection and insulation reliability.
In order to achieve the above object, the present invention has the following structure. Namely, it is a vacuum switch fitted with a vacuum valve having
an insulation tube to both ends of which is fitted an end cap;
a fixed-side conductive rod, which is fitted loosely into one side of the said insulation tube and the base of which is fixed to the said end-plate and to the tip of which is fixed the fixed-side contact: and
a movable-side conductive rod which is fitted loosely into the other said end-plate via a bellows and to the tip of which is fitted the movable-side contact;
the vacuum switch comprising:
a circular third electrode fitted coaxially at the intermediate part of the inner periphery of the said insulation tube, opposite the said fixed-aide contact and the said movable-side contact.
By these means, in the present invention, any arc generated when the contact is opened is led through the two gaps formed linearly between the third electrode and the fixed-side contact and moveable-side contact and this reduces the differences in interruption characteristics caused by differences in dielectric breakdown voltage.